This invention relates to a differential amplifier with common-mode rejection, comprising a first through fourth transistor, each having a first main electrode, a second main electrode and a control electrode, the differential amplifier further comprising a current mirror having an input terminal, a first output terminal and a second output terminal, in which:
the first and the third transistor each have their control electrodes coupled to a first input terminal of the differential amplifier,
the second and the fourth transistor each have their control electrodes coupled to a second input terminal of the differential amplifier,
the first through the fourth transistor have their second main electrodes coupled to a first supply terminal,
the first and the second transistor have their first main electrodes coupled, respectively, to the first and the second output terminal of the current mirror, and
the third and the fourth transistor have their first main electrodes coupled to the input terminal of the current mirror.
Such a differential amplifier is known, inter alia from U.S. Pat. No. 5,032,797. In this known differential amplifier the first and the second transistor and the third and the fourth transistor form a first differential pair and a second differential pair, respectively, which first differential pair has its output terminals coupled to the output terminals of a current mirror and which second differential pair has its output terminals coupled to an input terminal of the current mirror. The second differential pair in conjunction with the current mirror provides common-mode rejection at the output terminals of the first differential pair, which output terminals form the output terminals of the differential amplifier. However, in the known differential amplifier the tails of the differential pairs each include a transistor for transconductance control of the differential pairs.
A drawback of such a transconductance control is that a voltage drop is produced across said transistor. This is not favourable for use at low supply voltages. However, when the transistors in the tails of the differential pairs are omitted the d.c. level at the output terminals of the differential amplifier will become dependent on the common-mode current because this current is no longer maintained substantially constant by the transistor in the tail of the second differential pair.